The state-of-the-art method for electrolyzing an alkali metal halide, especially sodium chloride (NaCl) or potassium chloride (KCl), is to use a fluorinated ionomer membrane to separate the anolyte and catholyte compartments of an electrolytic cell. The membrane permits the alkali metal cations to pass through to the catholyte, but severely restricts the undesirable passage of hydroxyl ion from the catholyte to the anolyte. To make membrane electrolysis attractive, the power consumption should be minimized, which means that the current efficiency should be maximized and the cell voltage (or resistance) should be minimized.
Many efforts have been made to improve the performance of electrolytic cells and fluorinated ionomer membranes by a wide variety of treatments. Many of them have been aimed at higher current efficiency and lower power consumption. However, it is also desirable to obtain lower voltage. Any voltage over that needed to electrolyze brine is lost as heat and represents a waste of electric power. Excessive heat production can limit electrolyzer productivity by raising cell temperatures and increasing gas volume. Also, the rectifiers used in chloralkali plants are rated for power, which is the product of voltage and amperage. At higher voltages, less amperage can be supplied, reducing the productivity of the electrolyzers.
The art which is believed to be closest to the present invention is in Japanese Patent Application Publication 554-99797 (Toshio Oku, et al., Tokoyama Soda Company, Ltd.). These inventors reduced cell voltage by adding a water soluble substance of at least 100 molecular weight to the catholyte. The water soluble substances described in the application include polyvinyl alcohol, polyether, surfactant, gelatin, water-soluble cellulose, sugars and agars, which are present in the catholyte chamber at an effective concentration not exceeding 1%, preferably 10-100 ppm. The inventors indicate that they believe that the cell voltage is reduced because the state of foam inside the cathode chamber is altered, changing from a turbid suspension containing hydrogen gas to separate phases, one being a foam of relatively large particle size and the other being a clear liquid. However, the additive can be degraded by the reactants and must be continuously or frequently added to the cathode chamber to achieve the desired reduction in cell voltage. Furthermore, the additive does not result in a permanent membrane treatment which can survive harsh electrolysis conditions, including shutdown and start-up of the electrolytic cell, and storage of the membrane.